1. Field of the Invention
The present invention relates to an interface circuit for audio signal.
2. Description of the Related Art
The cost of an IC increases with an increase in the number of terminal pins. Further, when the number of terminal pins increases, the number of external parts also increases. In such state, the layout of the wiring pattern of the printed board on which the IC and external parts are mounted becomes complicated and, hence, the cost of the overall device including the printed board and external parts increases.
Further, the increase in the number of terminal pins makes miniaturization of the IC package unachievable and, combined with the increase in number of external parts, prevents downsizing of the apparatus using ICs. Furthermore, when there are a large number of terminal pins, such troubles as solder bridging easily occur during the mounting of the IC, and this also leads to an increase in cost.
Therefore, the increase in the number of terminal pins brings about a great disadvantage for ICs and apparatus using ICs.
Now, circuits around an AM detector and an audio amplifier a subsequent stage of a radio receiver are arranged in an IC for example, as shown in FIG. 4.
Referring to FIG. 4, reference numeral 10 denotes a one-chip IC for an AM receiver, in which an intermediate-frequency signal Si from an external ceramic filter 4 is supplied to an amplifier 21 through a terminal pin T7 and the intermediate-frequency signal Si from the amplifier 21 is supplied to an AM detector 22 to be subjected to AM detection therein, and thereby, an audio signal Ss and a D.C. component V22 at a level corresponding to the level of the intermediate-frequency signal Si are obtained. Then, the audio signal Ss of the detection output is supplied to an audio amplifier 23 through a signal line passing through a terminal pin T12.fwdarw.an external capacitor C11.fwdarw.a terminal pin T13. The signal Ss from the amplifier 23 is supplied, through a terminal pin T8 and a DC blocking capacitor C5, to a speaker SP.
At the same time, the detection output of the detector 22 is supplied to an AGC voltage generator 24 and, therein, an AGC voltage is generated from the DC component V22. This AGC voltage is supplied to the amplifier 21 as the control signal for its gain and, thereby, an AGC operation is performed on the intermediate-frequency signal Si. For this purpose, the generator 24 is connected to capacitor C4 through a terminal pin T6. This capacitor C4 forms a low-pass filter and the DC voltage 22 is extracted thereby from the detection output and the AGC voltage is generated from this DC voltage V22.
In the arrangement of FIG. 4, the capacitor Cll is provided for blocking the DC-component V22 included in the detection output of the detector 22 and also for supplying the AC component (audio signal) to the amplifier 23 and, therefore, the capacitor C11 is required to leave a capacitance of several .mu.F. Hence, the capacitor Cll cannot be arranged in the IC but has to be externally attached to the IC 10 as shown in FIG. 4.
Further, the capacitor C4 is provided for extracting the DC component V22 included in the detection output of the detector 22 as the AGC voltage and, hence, it has to have a capacitance of several .mu.F. Therefore, this capacitor C4 also cannot be arranged in the IC but has to be externally attached to the IC 10 as shown in FIG. 4.
Then, by such provision of the capacitors C11 and C4 external to the IC 10, the number of the terminal pins of the IC 10 i% increased and the above mentioned problems are created.